A typical chewing gum base includes one or more elastomers, one or more fillers, one or more elastomer solvents, softening agents and optional plastic polymers and miscellaneous colors, flavors and antioxidants. Due primarily to the difficulty in melting and dispersing the elastomers homogeneously among the other gum base ingredients, gum base manufacture has typically been a tedious and time-consuming batch process. For example, one such conventional process uses a sigma blade batch mixer having a front to rear blade speed ratio of 2:1, and a mixing temperature of about 80-125.degree. C.
In this conventional process, initial portions of elastomer, elastomer solvent and filler are added to the heated sigma blade mixer and blended until the elastomer is melted or smeared and thoroughly mixed with the elastomer solvent and fillers. Then the remaining portions of elastomer, elastomer solvent, softening agents, fillers and other ingredients are added sequentially, in a stepwise fashion, often with sufficient time for each stepwise addition to become completely mixed before adding more ingredients. Depending on the composition of the particular chewing gum bases and, in particular, the amount and type of elastomer, considerable patience may be required to insure that each ingredient becomes thoroughly mixed. Overall, anywhere from one to four hours of mixing time can be required to make one batch of chewing gum base using a conventional sigma blade mixer.
After mixing, the molten gum base batch must be emptied from the mixer into coated or lined pans, or pumped to other equipments such as a holding tank or a filtering device, then extruded or cast into shapes, and allowed to cool and solidify, before being ready for use in chewing gum. This additional processing and cooling requires even more time.
Various efforts have been undertaken to try to simplify and reduce the time required for gum base manufacture. European Patent Publication No. 0 273 809, in the name of General Foods France, discloses a process for making nonadhesive chewing gum base by blending elastomer and filler components together in an industrial mill type mixer to form a nonadhesive premix, dividing the premix into fragments, and blending the premix fragments and at least one other nonadhesive gum base component together in a powder mixer. Alternatively, the premix fragments and other base components can be added to an extruder along with other chewing gum components to accomplish direct manufacture of chewing gum.
French Patent Publication No. 2 635 441, also in the name of General Foods France, discloses a process for making a gum base concentrate using a twin screw extruder. The concentrate is prepared by mixing high molecular weight elastomers and plasticizers in desired proportions and feeding them into the extruder. Mineral fillers are added to the extruder downstream of the feed inlet of the elastomer/plasticizer blend. The resulting gum base concentrate has a high level of elastomers. The concentrate can then be mixed with the other gum base ingredients to provide a complete gum base.
U.S. Pat. No. 3,995,064, issued to Ehrgott et al., discloses the continuous manufacture of gum base using a sequence of mixers or a single variable mixer.
U.S. Pat. No. 4,187,320, issued to Koch et al., discloses a two stage process for preparing a chewing gum base. In the first stage, a solid elastomer, an elastomer solvent, and an oleaginous plasticizer are combined and mixed together under high shear. In the second stage, a hydrophobic plasticizer, a non-toxic vinyl polymer, and an emulsifier are added to the mixture and mixed using high shear.
U.S. Pat. No. 4,305,962, issued to Del Angel, discloses an elastomer/resin masterbatch formed by mixing a finely ground ester gum resin with a latex elastomer to form an emulsion, coagulating the emulsion using sodium chloride and sulfuric acid, separating the coagulated solid crumbs from the liquid phase, washing the solid crumbs, and removing the excess water.
U.S. Pat. No. 4,459,311, issued to DeTora et al., discloses making gum base using two separate mixers--a high intensity mixer for pre-plasticizing the elastomer in the presence of a filler, followed by a medium intensity mixer for ultimately blending all the gum base components together.
U.S. Pat. No. 4,968,511, issued to D'Amelia et al., discloses that chewing gum can be made directly in a one-step compounding process (without making an intermediate gum base) if certain vinyl polymers are used as the elastomer portion.
Several publications disclose that a continuous extruder can be used to make the ultimate chewing gum product after a separate process has previously been used to make the chewing gum base. These publications include U.S. Pat. No. 5,135,760, issued to Degady et al.; U.S. Pat. No. 5,045,325, issued to Lesko et al., and U.S. Pat. No. 4,555,407, issued to Kramer et al.
Notwithstanding the prior efforts described above, there is a need and desire in the chewing gum industry for a continuous process which can effectively and efficiently be used to make a variety of complete chewing gum bases without limiting the type or quantity of elastomer employed, and without requiring preblending or other pretreatment of the elastomer.
Continuous gum base manufacturing processes, while desirable, present a number of difficulties. One of these is that continuous equipment has a given processing length once set up for operation. This length is limited in practice by what is commercially available, and is often less than what may be desired from the gum base manufacture's standpoint. As a result, continuous mixing operations have less degrees of freedom than traditional batch processes. For example, in a batch process, if longer mixing times are needed, it is a simple matter to continue mixing. However, the residence time in a continuous mixer is a function of the operating speed and feed rates. Therefore, to change the mixing time, some other factor must be adjusted and accommodated. Further, in a batch process, additional ingredients can be added at any time. Commercial continuous mixers have a limited number of feed inlets at fixed positions. Therefore the additional ingredients can be added at only preset points in the mixing process.
Also, in a batch mixer, dispersive and distributive mixing can be independently varied and controlled. On a continuous mixer, changes to one type of mixing will often also affect the other type of mixing. If the amount of the machine used for high shear mixing is increased, there is less machine available for distributive mixing. Also, if the speed is increased, heat may be generated beyond the ability of the cooling capabilities of the equipment.
One of the particular problems that has been encountered during development of continuous gum base manufacturing processes is that the properties of the chewing gum base, particularly the softness of the chew, is a function of the gum base ingredients and the mixing conditions that are applied to those ingredients. However, the mixing conditions are also a function of the gum base ingredients, as well as the type of mixing elements being used, the temperature and viscosity of the ingredients and the fullness of the mixer barrel. For example, if there is a high content of filler in the base, more aggressive mixing occurs in the mixer because the filler acts as an abrasive. Conversely, if the filler level in the gum base is low, the mixing is less aggressive, and may not produce sufficient dispersive mixing of the elastomer.